Problem

[Artur]

I'm apologize but but I was do mistake in formula
Find number m such that 5n^4(4+n^2) is square
Mathemtica sample Code:
a={};Do[If[IntegerQ[Sqrt[5n^4 (4+n^2)]],AppendTo[a,n]],{n,1,1364}];a

Out: {1,4,11,29,76,199,521,1364}

Artur pisze:
> Who have idea how solve folowing problem
>
> Find number m such that 5n^4(4+n)^2 is square
> or Sqrt[5] n^2(4+n) is integer
> a(n)= {1,4,11,29,76,199,...}
> I need ideas how find a(21) and more
>
> Artur
>
>